In mobile devices and small-sized battery-driven electronic devices, a power supply using a battery is used for driving electronic circuits for driving a sensor and controlling and processing sensor signals. For electronic components and magnetic sensors to be used in the electronic devices, power voltage reduction of the electronic power consumption has been demanded.
In a new application of a non-contact high-accuracy rotary sensor, a weak magnetic field sensor, a direction sensor using earth magnetism, and detection of a magnetic ink print pattern by detecting a weak magnetic field caused by magnetic ink, a Hall element and a magneto resistance element are required to satisfy the demands of high performance and high reliability, and high sensitivity for magnetic fields, high input resistances, and very small sizes. Moreover excellent flatness on the device surfaces, high element manufacturing accuracy, and less variation in performance and less variation in manufacturing accuracy are demanded. Further, lower dependency on temperatures around room temperature is also demanded. Such demands are many, and the demanded specifications are strict. Further, a micro size magnetic sensor has been demanded.
Particularly recently, application of a magnetic sensor such as a Hall element is being studied and the magnetic sensor is required to satisfy the demand of sensitivity for detection of ultraweak magnetic fields of microtesla or nanotesla in detection of ultra fine magnetic particles of 1 micron or submicron.
1) High sensitivity (large μH), 2) less temperature dependency (a donor impurity-doped, for example, Sn-doped operation layer=Sn-doped conduction layer), 3) low power consumption (high input resistance=thin operation layer), and 4) micro size device have been demanded.
To manufacture a magnetic sensor which satisfies the above-described demands, InSb having the highest electron mobility is optimal. Further, a thin-film material which has high electron mobility enabling manufacturing of a magnetic sensor with high magnetic field detection sensitivity and enabling manufacturing of a high-input resistance magnetic sensor that can be driven with very small voltage and current, that is, an InSb thin film which has a great sheet resistance and is, accordingly, very thin is required.
Conventionally, it is common that an InSb thin-film magnetic sensor such as a Hall element or a magneto resistance element is manufactured by using an InSb thin film obtained by directly growing InSb on an insulating substrate of GaAs, etc. However, when a magnetic sensor is manufactured by using this InSb thin film obtained by directly growing an InSb thin film on a GaAs substrate, if the electron mobility is increased to increase the sensitivity in a magnetic field, as a result, the electric conductivity increases and the sheet resistance lowers. Further, if the film thickness of InSb is increased as a means for improving the crystallinity to have large electron mobility, the sheet resistance lowers again.
Further, InSb must be doped with a donor impurity such as Si and Sn for the purpose of improvement in reduction of temperature dependency, however, all of these means lower the element resistance, and in conventional magnetic sensors having a magnetic sensitive portion formed of an InSb thin film, the element resistance inevitably lowers if the performance such as sensitivity is improved, and it is very difficult to realize both higher sensitivity and higher element resistance.
This problem is solved by thinning the InSb thin film of the element portion and increasing the sheet resistance, however, there is no substrate that has the same lattice constant as that of InSb and is insulative. Therefore, when an InSb thin film is grown on an insulative GaAs substrate, film thickness dependency of the performance such as the electron mobility is extremely great, and further, the crystallinity of the InSb thin film rapidly lowers with the film thickness reduction. As a result, when the InSb film thickness is small, the performance of the InSb thin film directly formed on the GaAs is very poor, and it is very difficult to make this thinner without performance deterioration.
This is caused by a difference in the lattice constant between the substrate on which the InSb thin film is grown and InSb, that is, great lattice mismatch between InSb and the substrate. GaAs and InP substrates are used as insulative single-crystal substrates to be normally used for growth of an InSb thin film thereon, however, these substrates are greatly different in the lattice constant from InSb. For example, the difference in the lattice constant between GaAs and InSb is 14%. There is no group III-V compound semiconductor whose lattice constant is equal to that of InSb. Therefore, GaAs substrates, InP substrates, Si substrates, and sapphire substrates have been used as substrates on which a single-crystal thin film of InSb is grown although lattice constants of these substrates are greatly different from that of InSb.
An attempt to eliminate such a great difference in the lattice constant from the substrates was made. For example, in Patent Document 1, to eliminate the difference in the lattice constant from InSb of an active layer (to match the lattice), a structure in which an InSb buffer layer doped with an acceptor impurity is placed on the substrate, an undoped InSb active layer is further formed, and next, an impurity is doped and a strained AlGaInSb carrier supply layer whose lattice constant in the horizontal direction with respect to the substrate surface is equal to the lattice constant of InSb is formed, and further, on this layer, an undoped InSb cap layer is formed, is proposed, however, as this structure is very complicated, it is difficult to be practically manufactured. The performance of the InSb layer in this structure in the case where a practical element is not manufactured seems good, however, this includes a great problem in the case where a practical element is manufactured.
In other words, in this structure, an undoped InSb layer that is a conductive layer made of the same material as the operation layer is formed on the top in addition to the operation layer. When an element such as a magnetic sensor is manufactured with this structure, normally, an insulating film is formed on and in contact with this conductive layer, however, it is known that the lattice of the InSb thin layer is broken due to lattice mismatch with the insulating film and impact when forming the insulating film, and this results in performance deterioration such as a carrier increase in the InSb layer and extreme lowering in electron mobility.
When a Hall element or a magneto resistance element is manufactured with this structure, the deteriorated InSb layer becomes a simple leak layer of the drive current of the element, the drive current flowing in the operation layer is branched and the current flowing in the operation layer becomes substantially small. This leak layer current remarkably deteriorates the performance such as magnetic field sensitivity. Therefore, this structure is unsuitable for manufacturing a magnetic sensor such as a practical Hall element, so that it is difficult to manufacture practical Hall element and magneto resistance element.
Further, in contact with the undoped InSb layer as an operation layer, formation of an acceptor impurity-doped InSb layer with the same lattice constant is essential. Provision of insulation at room temperature by doping an InSb layer that is an intrinsic semiconductor at room temperature or a higher temperature with an acceptor impurity is generally very difficult due to a small band gap (0.17 eV), so that it is impossible to obtain high insulation and high resistance. At room temperature or a higher temperature, in an InSb that does not serve as a p-type, it is impossible to obtain electric insulation by pn junction.
Thus, in the technique of Patent Document 1, a very complicated structure is necessary to eliminate the lattice constant difference and manufacture a high-performance InSb thin-film operation layer, and InSb that is not an operation layer must also be formed on the top of the structure. This construction has a great problem at room temperature or a higher temperature at which the magnetic sensor is used, and particularly, manufacturing of a practical InSb magnetic sensor usable across a wide temperature range from a low temperature to a high temperature is very difficult. Manufacturing of a practical magnetic sensor usable as a magnetic sensor, etc., installed in a vehicle, required to stably operate in a range of −40 to 150° C. or higher has not been realized yet.
It is conventionally impossible to manufacture a very thin film of InSb and manufacture a high-sensitivity magnetic sensor such as a Hall element by using the conventional technique. Particularly, a technique for manufacturing a high-sensitivity magnetic sensor by obtaining high electron mobility in a very thin InSb single-crystal thin film with a thickness of 0.5 micrometers or less or 0.2 micrometers has not been found yet.
Therefore, the present inventors examined a high-sensitivity, high-resistance, and practical InSb magnetic sensor using an InSb single-crystal thin film which the conventional technique had not realized as an operation layer, and a method for manufacturing this sensor.
It would be convenient if an insulating substrate of a group III-V compound semiconductor lattice-matched with InSb could be manufactured, however, such a substrate does not exist. Therefore, in manufacturing of an InSb thin film, lattice mismatch with the substrate is a great problem. Therefore, it is necessary to create a technique for manufacturing an InSb single-crystal thin film with excellent crystallinity and high electron mobility regardless of this lattice mismatch, and this is an object of the present invention.
Therefore, the inventors have taken on the challenge of researching a crystal growing method by which an InSb thin film could be obtained without the preconditions of lattice matching. That is, they researched a crystal growing technique by which a thin film with excellent performance could be obtained by growing the single crystal of InSb regardless of lattice mismatch with the substrate on which InSb is grown. As a result, they found that InSb being excellent in quality although its thickness was small grown by growing a group III-V mixed crystal layer satisfying special conditions in which it was insulative and had smaller mismatch although its lattice mismatches with InSb, on a substrate, and growing InSb on this mixed crystal layer by means of molecular beam epitaxy (MBE).
That is, it was found that, even if the mixed crystal layer with which the InSb thin film was in direct contact and InSb had a lattice mismatch with each other, when the lattice mismatch was in a certain range, and the composition and crystallinity of the mixed crystal layer satisfied predetermined conditions, the performance of InSb to be grown on the mixed crystal layer was excellent.
For example, when InSb is directly grown on a GaAs substrate, the lattice mismatch is 14%, and in a case of an InSb thin film with a thickness not more than 1 micrometer, high electron mobility cannot be obtained even if it is a single-crystal thin film. Further, with a reduction in film thickness to 0.5 micrometers, and even further, to 0.2 micrometers, the electron mobility of InSb rapidly lowers. This is shown by the experimental data indicated by the line of square marks in FIG. 10.
FIG. 10 is a graph showing a relationship (triangle marks) between the film thickness and electron mobility of the InSb thin film directly formed on a GaAs substrate.
From the data of FIG. 10, it is known that, when the thickness of the InSb thin film is 0.1 micrometer, a very small value of 3,000 cm2/Vs is observed, and manufacturing of a high-sensitivity magnetic sensor is difficult. This is an inevitable result from the 14% lattice mismatch. The electron mobility of InSb grown on this GaAs is also shown in Non-Patent Document 1.
The above-described structure of Patent Document 1 eliminates lattice mismatch by using a p-type or insulating InSb (doped) layer formed on an AlGaInSb layer as a buffer layer, and the film quality of InSb (undoped) as an operation layer formed on the buffer layer is secured.
According to Non-Patent Document 1, in the InSb thin film directly formed on the GaAs substrate, the existence of a low-electron mobility layer formed in the vicinity of a hetero interface between the GaAs substrate and InSb due to lattice mismatch based on a 14% difference in the lattice constant between the GaAs substrate crystal and InSb, and the existence of a low-electron mobility layer naturally formed on the InSb thin film surface are described. Due to low-electron mobility layers on both surfaces of the InSb thin film, the electron mobility becomes smaller (lower) as the InSb film thickness becomes smaller. Particularly, when the InSb film becomes thinner than 0.2 micrometers, the electron mobility lowering of the InSb thin film is remarkable, and manufacturing of an InSb Hall element with practical sensitivity is conventionally difficult.
This electron mobility lowering of the InSb thin film according to film thickness reduction corresponds to the existence of a low-electron mobility layer produced at the hetero interface with the GaAs substrate and the thickness thereof during crystal growth. The thickness of this low-electron mobility layer is generally 0.1 to 0.2 micrometers although this depends on the crystal growth conditions. To increase the electron mobility of the InSb thin film at very small thin film thickness, it is necessary to reduce or eliminate the thickness of the low-electron mobility layer.
According to Non-Patent Document 1, it is known that the InSb thin film grown on the GaAs substrate greatly changes in electron mobility and electron concentration in the thickness direction, and describing based on a simple model of distribution of properties of the thin film, these changes consist of a low-electron mobility layer which is in contact with the hetero interface with the substrate and is grown first (layer which has many defects due to lattice mismatch with the substrate and is not excellent in physical properties), and a high-electron mobility layer (layer which is not influenced by mismatch and is improved in physical properties and have very few defects).
If the high-electron mobility layer with high electron mobility is thick, that is, by reducing the thickness ratio of the low-electron mobility layer, the electron mobility of the InSb thin film increases, and a magnetic sensor such as a high-sensitivity Hall element can be manufactured.
The high-electron mobility layer can be easily increased in thickness by simply increasing the InSb film thickness, however, in this case, an input resistance becomes smaller when a magnetic sensor is manufactured, and this poses a problem such as an increase in power consumption of the magnetic sensors, and is disadvantageous in practicability.
To increase the input resistance, the InSb thin film must be thinned, however, in this case, the high-electron mobility layer becomes extremely thin or, depending on circumstances, it completely disappears, so that an InSb thin film with high electron mobility cannot be obtained. For example, with a film thickness smaller than 0.2 micrometers, most of the portion with high electron mobility disappears. Even with the thickness of 0.3 micrometers, the low-electron mobility layer becomes thicker than the high-electron mobility layer, and as a result, the electron mobility is not as high as expected.
Thus, when the InSb film thickness is not more than a thickness substantially corresponding to the thickness of the low-electron mobility layer formed in the vicinity of the hetero interface or occupies 50% or more of the film thickness, the electron mobility extremely lowers, and therefore, it is conventionally impossible to manufacture a practical magnetic sensor such as a high-sensitivity Hall element. This is also obvious from the relationship between the film thickness and the electron mobility when an undoped InSb thin film is directly grown on a GaAs (100) substrate as shown in Non-Patent Document 1 and FIG. 10.
When the film thickness of InSb is thus reduced, the layer with high electron mobility described in Non-Patent Document 1 becomes extremely thin, and most of the InSb thin film lowers in electron mobility, so that the electron mobility of the entire InSb thin film rapidly lowers.
Even when the substrate changes from GaAs to other substrates, the same phenomenon occurs if the film has lattice mismatch with the substrate. Thus, to manufacture a high-sensitivity magnetic sensor, an InSb thin film having great electron mobility is essential, however, if the film thickness of InSb becomes small, the electron mobility rapidly lowers. Therefore, conventionally, with an extremely thin InSb film, a thin-film lamination including a conductive layer of InSb used as an operation layer of a magnetic sensor capable of detecting magnetic fields with high sensitivity and an InSb magnetic sensor using this thin-film lamination in the magnetic sensor section cannot be manufactured.
When the InSb film thickness is reduced, a high sheet resistance can be expected, however, the electron mobility which determines the sensitivity of the magnetic sensor becomes extremely low. For example, the electron mobility of an InSb thin film with a thickness of 1.0 micrometer directly formed on a GaAs (100) substrate exceeds 50,000 cm2/Vs, however, the electron mobility of an InSb thin film with a thickness of 0.3 micrometers is about 20,000 cm2/Vs, and in the case of a film thickness of 0.2 micrometers, the electron mobility is not more than 10,000 cm2/Vs, and in the case of an InSb thin film with a thickness of 0.15 micrometers, the electron mobility is about 7000 cm2/Vs or less, and in the case of 0.1 micrometers, the electron mobility is not more than 5,000 cm2/Vs or less, so that the electron mobility rapidly lowers according to film thickness reduction. From this fact, it is understood that the thickness of the layer portion with low electron mobility of the InSb thin film is 0.15 to 0.2 micrometers.
Thus, the electron mobility rapidly lowers with an InSb film thickness reduction, and reaches an extremely small value. Therefore, the sensitivity of the magnetic sensor using this InSb thin film in the sensor section rapidly lowers with the InSb film thickness, and a magnetic sensor such as a high-sensitivity practical Hall element or magneto resistance element cannot be manufactured. Thus, in the example of the InSb thin film directly formed on the GaAs substrate, it is known that the electron mobility greatly changes according to the film thickness of InSb due to a great lattice mismatch, that is, the electron mobility rapidly lowers with a film thickness reduction.
Reiterating once again, particularly, when the film thickness is reduced to 0.2 micrometers or less, due to the effect of lattice mismatch, the electron mobility of the InSb thin film formed on the GaAs substrate rapidly lowers. This is caused by great changes in properties of the InSb thin film grown on the GaAs substrate in the film thickness direction. Therefore, in the conventional technique, there is no thin film suitable with a thickness not more than 0.2 micrometers for manufacturing a practical high-sensitivity Hall element or magneto resistance element. However, a magnetic sensor such as an InSb Hall element or magneto resistance element with a high input resistance using an InSb thin film for the magnetic sensor section or portion is extremely important in terms of application. The needs for this are also high, however, it is mentioned here that neither an InSb thin film nor a magnetic sensor having high electron mobility usable for a practical magnetic sensor were manufactured in the past.
Next, reasons for production of this low-electron mobility layer will be described.
One of the reasons is, in particular, the existence of a portion with a high lattice defect density in the vicinity of the hetero interface between the substrate and InSb. That is, through analysis of electron transfer unique to the InSb thin film, it was found that InSb had a great lattice mismatch with GaAs, and a layer with a thickness not more than 0.2 microns from the hetero interface with GaAs had a high lattice defect density, poor crystallinity of InSb, and low electron mobility, and as a result, it formed a layer with extremely low electron mobility, and the electron mobility thereof was as extremely low as several thousand or less (for example, refer to Non-Patent Document 1). As a result, it is inevitable that the electron mobility of the InSb thin film has great film thickness dependency, and as the InSb becomes thinner, the physical properties such as electron mobility important in manufacturing a magnetic sensor rapidly lower.
However, if an insulating substrate whose lattice constant is equal to that of InSb is obtained, this problem may be solved, however, it is known that an insulating or insulative material of a group III-V compound semiconductor does not exist. As a result, phenomenologically and logically, mass production on an industrial scale of an InSb thin film or a thin-film lamination with a thickness not more than 0.3 micrometers, further, not more than 0.2 micrometers from which a practical InSb thin film magnetic sensor such as an InSb Hall element or magneto resistance element with high sensitivity and high input resistance is likely to be manufactured, is impossible.
Further, as described in Non-Patent Document 1, a thin layer which has low electron mobility and an about 50 nanometer thickness was also found on the surface layer of the InSb thin film from a detailed examination of the phenomenon of electron transfer in the InSb thin film. The cause of this is considered that, when the substance is neither air in nor a vacuum, the lattice of the portion with a thickness of about 50 nanometers inside the InSb surface is strained and forms a thin layer with low electron mobility. This low-electron mobility layer on the surface portion also influences the film thickness dependency which lowers with the InSb film thickness reduction.
Therefore, with a thickness not more than the thickness added with the thicknesses of the two low-electron mobility layers formed at the hetero interface between the surface and the substrate, an InSb thin film with high electron mobility cannot be manufactured. The thickness of this is about 0.2 micrometers.
To manufacture a practical magnetic sensor such as a high-sensitivity InSb thin-film Hall element, the comparatively thick low-electron mobility layers formed in the vicinities of the interface between the InSb thin film and the substrate and the surface of the InSb thin film caused by the above-described various factors are a big problem. That is, it is a big obstacle or problem to manufacture a practical magnetic sensor using an extremely thin film of InSb as a magnetic sensor section, and elimination or reduction in thickness of the low-electron mobility layers to be extremely thin are extremely important technical issues that should be solved.
In practical use of a magnetic sensor such as a Hall element, high reliability has been conventionally demanded. That is, in industrial use or use as an in-vehicle sensor, provision of high performance in reliability relating to practical use such as improvement in reliability, durability, and environment resistance performance have been demanded. Therefore, it has been demanded that a protective film, that is, a passivation film for the InSb thin film surface is formed. In other applications, for example, in magnetic microparticle detection by using an InSb Hall element, it has been demanded that the sensor section of the InSb thin film is made to approach a distance of several tens of micron, micron, and still further, submicron to a measuring target and detects the magnetic field. Therefore, it is required that a protective film of the InSb thin-film surface is formed so as not to damage the InSb thin film or the surface thereof when measuring the magnetic field.
On the surface of the InSb thin film of the magnetic sensor section, for the purpose of securing reliability of the element or the purpose of relaxing the stress on the thin film caused by heat generated during heat curing of a package resin, etc., an insulating film, for example, an insulating film of Si3N4 or SiO2, etc., is formed. That is, a protective layer or a protective film is formed. This protective layer is different not only in the crystal lattice but also in the lattice constant from InSb, and further, in the case of manufacturing by plasma CVD, the surface of InSb is exposed to collision of plasma ions, and is frequently damaged. By way of exception, the protective layer may not be formed, however, in the manufacturing process of the magnetic sensor, this protective layer is ordinarily formed and is essential.
Thus, on the upper surface of the InSb thin film constituting the magnetic sensor section, formation of a protective film is needed. However, this formation of a protective film damages the thin film portion proximal to the surface of the InSb thin film constituting the magnetic sensor section, and results in great deterioration in the performance of the thin film, and desired magnetic sensor performance cannot be obtained. This is a process fluctuation and damage inevitably occurring in the process of forming the protective layer.
As the damage of the InSb thin film due to formation of the protective film, the sensitivity lowers by about 10% (electron mobility lowering of the InSb thin film) according to the insulating film formation when an InSb thin film with a thickness of 1 micrometer is manufactured as a Hall element, however, if the thickness is reduced, this lowering remarkably increases, and in the case of a thickness of 0.3 microns, this sensitivity lowering amount is 40% to 70% and more depending on circumstances.
Table 1 shown below shows the relationship between the lowering in electron mobility of the InSb thin film and the InSb film thickness when SiN with a thickness of 0.3 micrometers is formed on the surface.
TABLE 1InSb film thickness (μm)Electron mobility lowering (%)0.610%0.520%0.430%0.340%
Therefore, it is impossible to manufacture a desired high-sensitivity magnetic sensor. The reason for this is considered that the surface of the InSb thin film is broken due to an impact of the plasma CVD or lattice mismatch, and the electron mobility of the broken portion becomes lower, and a comparatively thick layer with low electron mobility is formed in the vicinity of the surface in the InSb thin film, and as a result, great performance deterioration in the InSb thin film is caused.
Formation of a protective layer and an impact when forming a protective layer forms a low-electron mobility layer with a certain thickness in the surface of the InSb thin film as a result, and causes deterioration in the performance of the element. This low-electron mobility layer has a thickness of 50 to 100 nanometers (0.10 to 0.05 micrometers) although this depends on the conditions of protective layer formation, and is thicker than the thickness of 50 nanometers of the low-electron mobility layer which is naturally formed on the surface of the InSb thin film. Therefore, to increase the electron mobility of the InSb thin film formed by epitaxial growth on the substrate, it is essential to make the InSb thin film as thin and small as possible or eliminate the low-electron mobility layer formed in contact with a hetero interface between the InSb thin film surface and the substrate.
A solution for such problems of performance deterioration in the InSb layer relating to the protective film formation has been demanded for a long time. That is, an InSb magnetic sensor structure which does not cause performance fluctuation in the process of manufacturing the InSb thin film as an element by avoiding deterioration in performance of the InSb thin film due to formation of insulation, that is, a protective layer on the surface of the magnetic sensor section or realization of an element structure which does not structurally cause performance deterioration in the InSb thin film has been demanded.
The present invention was made in view of these problems, and an object thereof is to provide a thin film lamination to be used for a micro InSb thin film magnetic sensor which can directly detect magnetic flux density with high sensitivity and whose power consumption are small, an InSb thin film magnetic sensor using the same thin film lamination, and a method for manufacturing the same.
[Patent Document 1] Japanese Patent Laid-Open No. 2000-183424
[Non-Patent Document 1] Journal of Crystal Growth, Vol. 251, pp. 560-564 and Vol. 278, pp. 604-609